In recent years, the quality of commercial audio broadcast signals as delivered by radio transmitters through atmospheric free-space has been eclipsed by the quality of stored program material, such as digital compact disk and audio tape technology. The quality differential of such stored digital program material over conventional analog frequency modulated (FM) broadcasting is so significant that there has been a market shift in listener preference to the stored digital program material. Further adding to this market shift is the increased degradation of FM signal quality, particularly in highly urban areas, due to multipath and noise.
Multipath degradation and the quality of FM signals are inherent problems with the line of site propagation of radio signals, such signals being subject to interference and fading from reflected copies of the signal, both narrowband and broadband. Such interference resulting from the simultaneous reception of multiple propagation paths between the transmitter and receiver is commonly referred to as multipath (MP), the different propagation paths having varying times-of-arrival, amplitude, and phase.
Digital communication systems involve the transmission and reception of binary information at a prescribed rate of transmission (bits/sec) over communication links which typically suffer various impairments due to environmental factors. One of the most difficult environments in which to achieve high quality digital radio communication is the mobile reception of atmospheric free space propagation in urban areas. The principal impairment in such environments arises from multipath, which causes the receiver to simultaneously receive signals via multiple propagation paths disposed between the transmitter and receiver. Receivers under such circumstances detect the summation of all paths, thus leading to significant signal disturbance In urban settings, tall buildings and the like act as strong MP reflectors, particularly in the very high frequency (VHF) region of from about 30-300 MHz, thereby causing the aforesaid multipath problems.
Generally speaking, digital communication systems involve the encoding, transmission, reception, and decoding of symbols. Symbols (or bauds) are analog waveforms, whose shapes represent patterns of one or more bits of digital information (e.g. binary 0s and 1s). The "baud rate" is the reciprocal of the "baud interval", the baud interval being the interval of time that corresponds to the duration of the symbol waveform, i.e. the baud rate equals 1 over the baud interval.
Typically, in digital systems the symbol waveforms are themselves generated by the additive combination of other analog waveforms (multichannel), which are altered or modulated in a manner which distinguishes between the presence of at least a binary zero and binary one (e.g. BPSK, M-ary, QPSK, etc.). The effect of multipath is to change the received waveform shapes so that, when detected, the correct bit pattern can no longer be satisfactorily decoded or demodulated. Thus, the characteristics of the symbol shapes or waveforms and the encoding/decoding processes are major factors in the achievement of a satisfactory Bit-Error-Ratio (BER) in a digital communication system of the multicarrier or multichannel type.
A simple approach to eliminate the problem of multipath (MP) in digital communication systems is to use a highly directional receiving antenna that rejects all propagation paths save one, the received path depending on the directional orientation of the antenna. However, this approach is impractical in mobile and highly diverse applications for, as will be recognized of one of skill in the art, it would be highly inconvenient and difficult to place directional antennas in automobile radios and the like. Thus, omni-directional antennas are preferable, especially in mobile transmission and reception environments.
The adverse effects of multipath (MP) on an isolated signal waveform may be grouped at least into the following three categories fading, dispersion, and intersymbol interference. Fading involves rapid amplitude variation as propagation paths constructively and destructively interfere, but may be controlled under certain circumstances with automatic gain control (AGC) circuits.
Dispersion is caused by time-varying phase disruption within and between bauds, and may be controlled under certain circumstances with an automatic equalizer. For example, a conventional tapped delay line equalizer is useful where it is necessary to select among or to adjust several attenuation (amplitude) and phase characteristics, the output being taken from a summing network which adds or sums the outputs of the taps. Such an equalizer is adjusted to the desired response (equalization of both phase and amplitude) by adjusting the tap contributions
Intersymbol interference (ISI) is caused by the interaction of one symbol (or waveform) with other symbols in time. The automatic equalizer, which may be used to correct phase dispersion, may compensate for adverse effects of intersymbol interference when the symbol shape of the interfering waveform is approximately identical to that of the desired channel. However, it is difficult and expensive to correct intersymbol interference from undesired propagation paths which represent different symbols of bit patterns that substantially precede or succeed the desired symbol in time.
Therefore, a conventional technique often utilized to combat the adverse effects of multipath-induced intersymbol interference is to increase the time duration of the symbol interval (or baud interval) to be much longer than the expected multipath delay. Typical expected multipath delays generally range from a maximum of about 5 to about 30 microseconds (is) for a VHF channel. When this technique is properly utilized, the only substantial intersymbol interference remaining is caused by the symbol interfering with itself, this remaining interference being correctable under certain circumstances with an automatic equalizer.
However, the increase in time of the baud or symbol interval leads to a decreased data rate. Therefore, in order to maintain a specified data rate, it is necessary to increase the number of data channels (or the number of carriers or waveforms) in order to make up for the loss in throughput caused by the reduced baud rate (or increased baud interval).
Thus, a conventional approach utilized in balancing the problems of reduced data rate and multipath (MP) is the use of increased symbol or baud intervals in combination with multiple data channels or multicarriers, each channel having a particular and unique symbol waveform. Unfortunately, the presence of multiple data channels and therefore multiple waveforms becomes a further source of interference, particularly in the presence of multipath. This additional interference due to the multiple channels or carriers eliminates under certain circumstances some of the performance gains achieved via the use of increased baud or symbol durations.
In such multichannel systems with increased baud intervals, the shapes and characteristics of the basis waveforms have significant influences on the BER, the basis waveforms being the unmodulated sequences representing the data carrier of each channel. Accordingly, there has been research into defining desirable characteristics of signal waveforms usable in such multichannel environments for producing superior performance. This research has often been conducted in combination with the use of conventional correlation receivers. In a conventional correlation receiver, a satisfactory received signal is one which satisfies the spectral confinement requirements of the particular application, and is characterized by predetermined crosscorrelation and autocorrelation properties.
The crosscorrelation property (or orthogonality) is measured between a single signal waveform in a set and all other members of the waveform set. Low crosscorrelation is important in multichannel carrier systems in order to ensure that the individual carriers may be recovered and recognized independent of one another. The crosscorrelation represents the degree to which a particular waveform is mathematically correlated with another waveform(s) in the set. The smaller the absolute value of the crosscorrelation between any two waveforms, the more unique the waveforms in the correlation sense. Therefore, an ideal signal set for a correlation receiver has a crosscorrelation of close to about zero at the sampling point among all pairs of the set or, in other words, is a set where the waveforms are mutually orthogonal. Good crosscorrelation properties are also required for satisfactory channel performance absent multipath because channels act as sources of interference to each other.
Good autocorrelation is of primary importance in multipath environments. The autocorrelation property of a particular symbol or waveform is measured only with respect to that particular waveform. In other words, the autocorrelation property of a particular waveform is unaffected by properties of other sequences or waveforms throughout the set. The autocorrelation is a measure of how unique a signal is when compared to itself in a correlation receiver when shifted in time by a positive or negative amount or time shift. Good autocorrelation characteristics are critical for good performance in multipath environments because reception in such environments requires distinguishing among similar signals with varying times of arrival. An ideal signal set with respect to autocorrelation is one where the autocorrelation for each signal is at a minimum (or has a low value) for substantially all positive and negative time shifts and is at a maximum for about zero offset or, in other words, for relatively no time shift at all.
Signal waveforms constructed from amplitude samples of unconstrained (or unshaped) and non-orthogonal noise sequences have been proposed and utilized in prior art communication systems (e.g. spread spectrum applications) In a similar manner, prior art systems have utilized prime polynomials to generate pseudo-random binary sequences (also known as PN or direct sequence) which are limited to the values +1 and +1. Such bi-valued systems do, however, possess somewhat noise-like properties to a limited extent.
The prior art method of Code Division Multiple Access (CDMA) utilizes long baud intervals in a plurality of digital data channels, each carrier being a binary sequence obtained from, for example, Gold codes or Rademacher-Walsh codes. CDMA systems are spread spectrum systems that use multiple binary-valued codes to achieve a higher throughput or increased capacity than a single spread spectrum code. CDMA codes generally must make a tradeoff between crosscorrelation and autocorrelation, but typically cannot satisfy acceptable characteristics with respect to both.
A primary disadvantage of CDMA is that it does not permit spectral shaping of the carrier(s) without significant destruction of the sequence properties Additionally, the number of different acceptable signals which may be generated by CDMA codes is limited by the bi-valued nature of such signals.
The prior art method of Coded Orthogonal Frequency Division Multiplexing (COFDM) also utilizes long baud intervals or symbol intervals and numerous narrowband channels. However, each channel is often a sinusoid of prescribed frequency and phase Therefore, the waveforms representing the different channels of COFDM do not have noise-like properties, and are of the narrowband type thus leading to susceptibility to multipath and less than desireable autocorrelation values. While COFDM generally permits spectral shaping so as to allow individual carrier amplitudes to fit under predetermined spectral masks, a disadvantage of COFDM is that, because multipath is frequency selective, the COFDM narrowband channels with frequencies that correspond to multipath nulls will be significantly disrupted as discussed later herein.
An additional disadvantage of COFDM is that the presence of a narrowband interference within the COFDM spectrum will cause excessive bit errors because of the COFDM narrowband channels. While Forward Error Correction coding (FEC) may be used to improve the performance of COFDM, this improvement is at the expense of increased implementation complexity. Additionally, carrier recovery in OFDM systems is difficult because of different frequencies undergoing different phase shifts.
Additionally, the use of filtered Trellis-Coded Modulation (TCM) to shape signal spectra for use in digital data communication systems with sinusoidal signal waveforms is conventional in the art. However, Trellis codes alter the nature of the waveform properties so that, after interacting with the filter, only those patterns which produce waveforms with the desired properties are allowed to be used. Unfortunately, this requires a significant reduction in user data rate which is, of course, undesirable because, among other reasons, additional bits must be used purely for code shaping. Furthermore, Trellis codes require highly complicated and expensive receivers, which have much higher complexities than do simple correlation receivers.
The use of broadband or wideband signal sets is known as a method of combating the adverse effects of multipath. Wideband waveforms have diffused frequency content. Thus, because multipath is a frequency selective phenomenon, disturbing some frequencies more than others, it has an extremely adverse effect on narrowband waveforms such as sinusoids. In fact, in certain types of multipath known as specular, certain frequencies are allowed no transmission whatsoever of information. In multipath (MP) environments, although each of the wideband waveforms in a set suffers some interference or loss of signal due to certain frequency content disturbances (in contrast to the sinusoid example where some frequencies may be unaffected), as a whole, the wideband waveforms are all still sufficiently recognizable so as to permit accurate symbol recovery and thus allow for significant advantages in the presence of narrowband interference.
In applications where spectrum compliance is not an issue, direct-sequence spread spectrum techniques which utilize noise-like waveforms are effective in combating multipath. However, existing techniques for constructing noise (or the more restrictive example of pseudo-noise) waveforms do not permit arbitrary constraints in the shape of their spectral response without significantly disrupting the resulting waveform properties. This is important because practical systems require band limiting filters or similar processing in order to stay within a fixed frequency allocation and/or reject particular narrowband interference. Furthermore, although the crosscorrelation is small in spread spectrum systems, it is generally non-zero and hence the signal waveforms act as interferers to one another even in the absence of multipath.
U.S. Pat. No. 5,278,826 discloses a method and apparatus for digital audio broadcasting and reception wherein a system is provided for transmitting and receiving through free space a composite signal consisting of a frequency modulated (FM) analog signal and a multicarrier modulated digital signal which is especially adapted to be resistant to multipath degradation. The FM signal and digital multicarrier modulated signal are fully coherent. The digital signal comprises a plurality of carriers having a maximum amplitude at least 20 dB below the unmodulated FM signal, preferably 30 dB below the FM signal. Unfortunately, the multicarriers making up the digital signal in this patent are narrowband in nature, each carrier or channel being a single tone which is phase modulated. A problem with such carriers is that multipath (MP) is a frequency selective phenomenon which alters or destroys some frequencies while letting others alone. Thus, narrowband carriers are extremely vulnerable to the adverse effects of multipath. Furthermore, the digital frequency spectrum in this patent is extremely close to the FM center frequency, thus resulting in interference between the FM and digital signals.
U.S. Pat. No. 4,403,331 discloses a method and apparatus for transmitting digital data over limited bandwidth channels, with a set of waveforms being mutually orthogonal to one another. The use of orthogonal channel waveforms (known as basis functions) and bi-phase data modulation in order to use a correlation-type multiple channel or multicarrier receiver is known in the art, as illustrated by this patent. This patent discloses a technique for determining eigenvectors for the basis functions which maximize the spectral occupancy of the carrier waveforms primarily by utilizing a longer baud interval. The basis functions of this patent in certain embodiments are based on a fixed number of sinusoids (which are not noise-like), and the system utilizes an optimization in the frequency domain. Unfortunately, this does not translate into good autocorrelation properties or resulting waveforms which may be made phase-continuous at the baud boundaries. The lack of phase continuity at baud boundaries increases intersymbol interference, thereby limiting the ability to properly receive signals with good BER. Additionally, it is noted that optimization in the frequency domain does not translate necessarily into optimization in the time domain.
It is apparent from the above that there exists a long felt need in the art for a digital data transmission system and method including the production of a set of wideband spectrally shaped noise-like waveforms which are mutually orthogonal to one another and have good autocorrelation values so as to enable an inexpensive and relatively simple correlation receiver to decode the digital data transmitted via the waveforms in the face of high multipath and intersymbol interference environments, the use of wideband noise-like orthogonal signals allowing the system to be robust (or strongly resistant) to multipath problems. Additionally, there exists a need for such a signal set which may be spectrally shaped so as to allow an analog signal (e.g. analog FM) to be disposed between two separate and spaced apart portions of the spectrally shaped orthogonal set of waveforms or sequences wherein each of the analog FM signals and the two surrounding digital waveform portions are disposed within a predetermined broadcast band or spectral mask.
There also exists a long felt need in the art for a system which allows a radio broadcasting station to simultaneously transmit the same program material via both analog FM and digital signals via a combined FM/digital signal to a plurality of radio receivers, any of which may receive and demodulate one of the FM and digital signals so that existing receivers will remain useful and receivers produced pursuant to the disclosed invention will be able to demodulate the digital signal.